The spectral properties of the synchrotron radiation produced by an undulator depend mainly on the quality of the undulator magnetic field: amplitude and phase of the magnetic field must be within small tolerances over the complete length of the undulator. The field quality is quantified by the so-called phase error. Techniques for phase error correction, the so-called shimming, of permanent magnet undulators with magnetized metal strips are state of the art. For superconductive undulators shimming with additional superconductive correction coils was proposed and experimentally tested. With these classical methods shimming is an iterative and time consuming process of measuring, applying shim coils and verifying the improved field quality.
Therefore a novel shimming concept based on superconductive closed-loops has been newly developed at LAS. This novel concept automatically minimizes field errors. The system works in the following way: high temperature superconductive closed-loops on top of the undulator coils enclose one or several undulator periods. In case each of the enclosed poles produces a perfect field (no phase error) the integral magnetic flux through the loops is zero and the induced current into the loop is zero. As soon as the field in one of the poles deviates from its ideal value a current is induced in the loop which compensates the field deviation.
This concept was verified in a proof of principle experiment. In the near future additional experiments on induction shimming with a complete short undulator prototype are planned.